How to Beat a Perfect Defense?

In my game I have trouble threatening my players because of their extensive use of perfects. In combat, they either use combos with perfects in them or perfects by themselves. Outside of combat they will use normal charms but I can only threaten them with being sniped so often. They have specifically taken charms to counteract unexpected attacks, shaping, and the like.

Well, the flaws of invulnerability would be the classical answer. If you are having trouble with people cheesing the conviction flaw, you can use the new errata found in Glories. That said, a prepared Solar is pretty darn close to invulnerable. Its one of the precepts of the setting. I suggest flurries to mote tap them. Even better if you throw a large number of attacks with a marginal, but existent chance of hitting. That way the players must decide if they can afford to perfect them.

In general, the massive expenditures on offense are not things to throw out regularly. A smart fighter will wait until his enemy slips up and then drop the combo of doom on them.

That said, you don't need to actually have a hit connect to make the players feel threatened. I have had fights where my players took no damage and still thought they were going to die.

drain them of motes that always makes it easier

You are threatening them.  Health levels of damage aren't meaningful given the game's lethality; one good hit will kill you.  The better analogue to HP is their motes.  They use a lot of motes on a turtle strategy in a fight?  They are extremely wounded.  That was dangerous.  They almost died.

In order to effectively fight an enemy with perfect defenses, you have to maximize the threat of your attacks while minimizing their cost. The attack should be dangerous enough for a perfect defense to seem like the most viable defense option against it, but cheap enough that you don't risk mote-tapping when trying to mote-tap them.

Things that help with that:
-Good passive Accuracy (if you can muster around 15 dice on your attack pool before Charms, odds are good the opposition will seriously consider perfect defending against it, unless they're Sidereals with crazy DV ratings and expensive perfect defenses)
-Good passive damage (few people want to risk taking a hit from a Grand Goremaul wielded by a Strength 5 Exalt)
-Infinite Mastery and it's counterparts
-Combat Charms that are activated in further steps of attack resolution, like Solar Hero Form, Life-Severing Blow or Ebon Dragon's Curses (the latter are the best since they are actually activated after the attack was resolved - if the players know their enemy can do this, they will get extra paranoid about even the weakest of his attacks).

All in all, just wait 'till they run out of motes and then you can finish them off with a Combo of doom. If you want to actually kill them, that is.

For a more general advice, remember that just because PCs don't lose health levels doesn't mean their lives aren't at stake in combat. If they come out of a combat scene unbruised but with just five motes and two willpower left, it means they barely did it. Exalted combat is a lot like wuxia movies where characters dance around each other perfectly avoiding all blows until someone gets tired and lets one blow through, at which point combat usually ends.

I've certainly felt the same thing, I feel like it's a pretty standard thing for Exalted storytellers (especially newer ones) to encounter. Ironically, I think the most frightening encounter I've subjected any of my players to came from some of the weakest enemies. One of my players, a one to two hundred XP combat built zenith nearly fell before four heroic mortals and a hundred XP dragon-blooded. The key is tactics. Planning. In just some random fight, or even worse, a fight they are prepared for, a PC solar is one of the most powerful things in the game. Cheap perfects, powerful attacks, large mote pools, and the ability to stunt makes them incredibly powerful. You need to give their enemies a plan. Either use a generic plan that makes it difficult for them (which is what I did, I basically hit him with Essence Disruption Technique and about six different poisons all in one tick), or have the antagonists spy on them, learn their weaknesses, where to hit to cause the most damage. Don't just arbitrarily give them this information, though, catching a spy in their midst could be a pretty sweet plot point, even if it's completely unplanned on your part.

Also, and I think this is one of the biggest things, don't underestimate your players. Do not shy away from throwing stuff at them that seems way too powerful. One of the great things about solars, is that with just a couple tools, they can last against anything. They may not have a chance to win, but I can guarantee you, I could stat an Essence 2 or 3 Solar who could survive against the Unconquered Sun for a good while. If you happen to overestimate them, they can survive and get away to fight another day. And even if you think you've overestimated them, there's a probably a good chance you haven't, and they'll come out ahead against all odds.

Ranged Dynast with Dragon Vortex and Pinions?

As has been said, the only way to beat a perfect defense is to keep the character from being able to use it in the first place, either by mote-tapping them (and, as has also been said, motes should be treated as hit points), or by using an attack against which they have no defense.

However, be sure you're doing it because the game needs it, not just because you're frustrated that you can't "beat" your players.  If they're enjoying being invincible, let them be.  Your job as ST is to give them an interesting story, not to whip the crap out of them.  If, however, they're all bored that all they ever do is perfect, then this is a viable field of inquiry.

Octopoid:

However, be sure you're doing it because the game needs it, not just because you're frustrated that you can't "beat" your players.  If they're enjoying being invincible, let them be.  Your job as ST is to give them an interesting story, not to whip the crap out of them.  If, however, they're all bored that all they ever do is perfect, then this is a viable field of inquiry.

I want to do it because I, as the storyteller, feel that my creative talents are being trivialized by perfect defenses.

When my players walk through a corridor full of a dozen rolling boulders madly smashing against each other and the wall I want them to come out the other side thinking to themselves, "Wow! That was a cool set piece!" not "Whatever. We're down 12 motes." I have this sincere sense that I can't get their adrenaline pumping because they never feel threatened.

Iry:

I want to do it because I, as the storyteller, feel that my creative talents are being trivialized by perfect defenses.

And as long as your players agree that this is a problem, and that some new solution needs to be implemented, then you're doing just fine.

Iry:

I want to do it because I, as the storyteller, feel that my creative talents are being trivialized by perfect defenses.

You may wish to try to have more interesting traps and obstacles in place instead of trying to beat perfect defenses.   Make a room with a dozen smashing boulders that absolutely pitch black and requires them to find a crystal that attached to one of the boulders before they can get out of the room. (Awareness and Athletics) Change the world around them with shaping effects. Puzzles that need to be solved by their characters (via Lore or Investigation).  
    Make your challenges more fantastic, and ones that aren't solved by perfect defenses.   

I prefer to take the Superman approach as well. Just because a character is invulnerable (personally) doesn't mean he cannot be harmed (indirectly).

I can see how this takes the fun out of dungeon-delving, since most of the traps are supposed to be a personal threat, but, hey, if invulnerability is proving a problem to you, look for inspiration in other creative works with invulnerable characters.

Also, make traps that grapple them.  Use villains who will diabolically destroy things they care about.  Make them start to seriously worry that they will be the last living things in a blighted wasteland, invincible and undying, but utterly broken and alone.

WhirlwindMonk:

You need to give their enemies a plan. Either use a generic plan that makes it difficult for them (which is what I did, I basically hit him with Essence Disruption Technique and about six different poisons all in one tick), or have the antagonists spy on them, learn their weaknesses, where to hit to cause the most damage. Don't just arbitrarily give them this information, though, catching a spy in their midst could be a pretty sweet plot point, even if it's completely unplanned on your part.

Essence Disruption Attack is broken as printed; it inflicts what is, if used intelligently, a game-ending restriction in any fight that would have been even without it, and provides no way to defend or negate it.

You can get a lot out of clever use of resources.